The long goal of this grant is to understand the mechanisms and regulation of mRNA turnover in yeast mitochondria. RNA turnover has been shown to play a key regulatory role in many biological processes. Mitochondrial gene expression in Saccharomyces cerevisiae has been found to be regulated primarily by nuclearly encoded factors. Most research in this area has focused on specific nuclearly encoded gene products that are involved in processing and accumulation of a particular mitochondrial mRNA such as CBP1 which is required for the expression of the mitochondrially encoded cytochrome b (COB) gene, and less on the degradation of mRNAs. In order to identify gene products involved in the degradation of COB mRNA, a mutant strain, soc1, was isolated in a suppressor analysis of temperature- sensitive cbp1 strains. This soc 1 mutation causes increased steady-state levels of COB mRNA without affecting the expression of a co transcribed tRNA, an increased levels of COX2, and ATP8/ATO6 transcripts. These results are suggestive that the gene product of SOC1 is required for the degradation of COB mRNA and may encode a mitochondrial nuclease. The phenotype of the soc1 mutant is the only mutant known to increase the levels of mitochondrial transcripts in such a manner. The specific aims of this proposal are as follows: (1)characterization of SOC1 by cloning and sequencing the gene, determining the subcellular location of the gene product, and constructing and characterizing deltaSOC1 strains. These studies will address the nature of the gene product of SOC1 and its effects on mRNA stability in yeast mitochondria. (2) isolation and characterization of other trans-acting factors involved in mitochondrial mRNA decay by selection of additional suppressors of existing soc1-1 strains and synthetic PET screens. These experiments will identify other components of the mRNA decay machinery (3) determination of the effects of the gene products involved in mitochondrial mRNA degradation on mRNA turnover rates by measuring decay rates in wildtype and mutant strains, and under different growth conditions that regulate mRNA levels in yeast mitochondria. This analysis is critical in determining whether a given mutation is responsible for changes in steady-state levels of mitochondrial mRNAs. In summary, this proposal addresses an issue that has not been studied in yeast mitochondria; that is, how yeast mitochondrial mRNAs are degraded, what components of the turnover machinery are required for mRNA decay, and how mRNA turnover regulates gene expression in yeast mitochondria.